Bearings are devices that permit relative motion between two parts. Rolling element bearings typically comprise inner and outer raceways and a plurality of elements (e.g. rolling elements such as balls or rollers) disposed therebetween. For long-term reliability it is important that the various elements have a high resistance to rolling contact fatigue, wear and creep. For these reasons, bearing steels are the material of choice for most bearing components.
Electric motors, generators and associated equipment are at risk when an electric current passes through a bearing. This can damage the contact surfaces of rolling elements and raceways in the bearing (electrical erosion) and rapidly degrade the grease. An additional risk in electric motors and generators comes from high frequency currents due to the inherent stray capacitance. The risk of damage increases if the application uses a frequency converter.
Electric currents in rotating shafts can be either intentionally established or produced by imbalances in inductive rotating machinery. In either case it is often desirable to insulate the two components from one another through an insulated bearing. The problem of shaft currents in rotating inductive machinery has been recognized for decades as a substantial factor limiting bearing life.
The contact areas between housing, outer ring, rolling elements, inner ring and shaft can act as electric contacts. Current flow through bearings can result in significant degradation of their useful life in a relatively short period of time. For example electric current passage will damage rolling elements and raceways of bearing rings and rapidly degrade the lubricant. Typical damage is characterised by craters and false brinelling.
There are several known approaches to eliminating current flow through bearings. Ceramic rolling elements have been considered for use in bearing applications. There are, however, perceived intrinsic limitations associated with the use of ceramic rolling elements in safety critical applications.
One approach to making a bearing less conductive is to alter the bulk material of bearing component so as to resist flow of electricity. This is limited by the mechanical and cost demands of bearing applications and manufacture. Alternatively, it is possible to coat bearing components with resistive materials, which is more cost effective. However, such coatings must be provided with sufficient hardness.
GB 1591560 discloses a machine supported by bearings disposed in bearing bores, wherein the bearing bores are insulated by a layer of alumina.
U.S. Pat. No. 4,320,931 discloses an insulated bearing wherein said bearing comprises a bearing shell and an inner bearing material (Babbitt). A base layer is applied to the shell (preferably by a plasma spray method), a thicker ceramic insulating material layer is disposed onto the base layer and finally an “adhesion” layer (preferably of a metallic material) is added to serve as a binder between the shell and the final layer of bearing material (Babbitt).
Insulating the housing or shaft is often expensive and time-consuming. U.S. Pat. No. 3,924,906 teaches an alternative method for insulating bearings. It discloses electrically insulated bearings comprising an insulating coating of a light metal oxide or ceramic. The insulating coating is applied to the non-rotating surfaces of the housing or bearings by plasma coating or flame spraying. The insulating coating is then coated with a varnish to impregnate the insulation and to prevent contamination. EP 1408249 discloses a similar method, without the varnish, wherein the surface of the bearing component also includes a tool reference plane utilizable for a process of finishing the electrically insulating layer or for a thickness control of the insulating layer.
Commercially available INSOCOAT® bearings (SKF) employ a similar method to that of U.S. Pat. No. 3,924,906. The outer (non-wear) surface of the outer ring of the bearing or the inner (non-wear) surface of the inner ring of the bearing are coated with a ceramic layer which is sealed with an acrylate-based composition. The coated surface is then machined to the required dimension.
It is an object of the present invention to provide an insulated bearing which can be fitted using standard methods and tooling and maintains its electrical performance even in high humidity environments for extended periods.
Accordingly, the present invention aims to tackle at least some of the problems associated with the prior art or, at least, to provide a commercially useful alternative.